When a girl's vagina is so incredibly tight that it is visible during sex. Her pussy lips stretch around the penis. The lips visibly get pushed in with each thrust and are pulled out when a cock is sliding out. Look it up on PornHub, it's a thing.

Guy 1: Dude, Melissa is so short and petite. I bet her pussy is so tight it grips.
Guy 2: I fucked her a while ago. My cock got a good pussy gripping. But not after I was done.